Revisiting Dynamic Processes and Relaxation Mechanisms in a Heterocyclic Glass-Former: Direct Observation of a Transient State

Despite decades of studies, a clear understanding of near-Tg phenomena remains challenging for glass-forming systems. This review delves into the intricate molecular dynamics of the small, heterocyclic thioether, 6-methyl-2,3-dihydro-1,4-benzodithiine (MeBzS2), with a particular focus on its near-Tg cold crystallization and relaxation mechanisms. Investigating isothermal crystallization kinetics at various temperatures reveals a significant interplay between its molecular dynamics and recrystallization from a supercooled liquid. We also identify two independent interconversion paths between energetically privileged conformers, characterized by strained transition states. We demonstrate that these spatial transformations induce substantial alterations in the dipole moment orientation and magnitude. Our investigation also extends to the complex salt PdCl2(MeBzS2), where we observe the transient conformers directly, revealing a direct relationship between their abundance and the local or macroscopic electric field. The initially energetically privileged isomers in an undisturbed system become less favored in the presence of an external electric field or ions, resulting even in an unexpected inversion of states. Consequently, we confirm the intramolecular character of secondary relaxation in MeBzS2 and its mechanism related to conformational changes within the heterocyclic ring. The research is based on the combination of broadband dielectric spectroscopy, X-ray diffraction, and quantum density functional theory calculations.


■ INTRODUCTION
It has been known for centuries that a wide range of liquids and melted materials, including organic nonpolymeric compounds, can undergo supercooling well below their typical freezing temperatures, ultimately forming a disordered glassy state. 1,2This process, commonly called vitrification, encompasses many intricate physical phenomena that remain incompletely understood. 3,4Rapidly increasing viscosity, dramatic slowing down of molecular motion (both translational and rotational) from microseconds to hundreds of seconds, intermolecular self-organization into clusters, cold crystallization from supercooled liquid state, and intramolecular conformational changes are only several examples of the physicochemical processes occurring near glass transition temperature (T g ). 4−18 For example, comprehensive investigations into a collection of more than 400 acylanilides have revealed a clear correlation between an increasing abundance of energetically similar conformers, lengthened crystallization time, and a diminished propensity for crystallization. 15−22 Finally, conformational alterations in organic glass-formers serve as an important source of intramolecular secondary relaxation processes in their dielectric response, which can be monitored by the broadband dielectric spectroscopy (BDS) technique. 5These dielectric processes, manifesting as step-like anomalies and loss peaks in the real and imaginary parts of the complex dielectric permittivity, occur in both the liquid and glassy phases and remain invariant to pressure changes. 5,23 single intramolecular secondary relaxation, labeled as the β process, has also been reported for 6-methyl-2,3-dihydro-1,4benzodithiine (abbreviated further as MeBzS 2 ) − a small heterocycle containing a six-membered thioether ring with two sulfur atoms. 24This compound is based on a rigid aromatic toluene-3,4-dithiol building block in which both sulfur atoms are linked by the ethylene bridge into the −S−CH 2 −CH 2 −S− moiety (Figure 1a).−27 MeBzS 2 itself is a moderately fragile glass-former, displaying low glass transition temperature (T g = 192 K) and melting point (T m = 266 K), as well as a considerable tendency toward recrystallization from its supercooled liquid state. 24Above its T g , MeBzS 2 is a conventional van der Waals liquid with negligible intermolecular selforganization tendency. 24Consequently, this small heterocycle constitutes a perfect model compound to study the cold crystallization phenomenon, which has not yet been performed on this representative.Regarding the dielectric response, MeBzS 2 is characterized by two relaxation processes in the dielectric permittivity representation: structural α relaxation and secondary β process. 24They have been associated with collective motions of entire molecules in a supercooled liquid and intramolecular conformational transformations within their heterocyclic ring.However, it is essential to note that determining the mechanism behind the β process in MeBzS 2 has relied solely on a single potential energy curve. 24In turn, the unambiguous assignment of secondary relaxation's mechanism is challenging in the case of heterocyclic compounds, as even in-plane 2D rotational motions should be considered. 28n this paper, we revisit molecular dynamics, near-T g cold crystallization, and relaxation mechanisms of the heterocyclic MeBzS 2 .Following isothermal crystallization kinetics at various temperatures, we uncover a significant interplay between molecular dynamics and cold crystallization from a supercooled liquid.Furthermore, we identify two independent interconversion paths between energetically privileged conformers characterized by strained transition states.We demonstrate that these spatial transformations induce substantial alterations in dipole moment orientation and magnitude.Our investigation also extends to the complex salt PdCl 2 (MeBzS 2 ), where we observe a transient conformer directly, revealing a direct relationship between their abundance and the local or macroscopic electric field.Consequently, we confirm the intramolecular character of secondary relaxation in MeBzS 2 and its mechanism related to conformational changes within the heterocyclic ring.The research is based on the combination of BDS, X-ray diffraction (XRD), and quantum density functional theory (DFT) calculations.
■ EXPERIMENTAL SECTION Materials.The object of the studies is an aromatic thioether with a six-membered heterocyclic ring, 6-methyl-2,3-dihydro-1,4-benzodithiine (MeBzS 2 ), and its complex salt PdCl 2 (MeBzS 2 ).Synthesis, purification, and basic character- The Journal of Physical Chemistry B ization of MeBzS 2 have been published by us previously. 24dCl 2 (MeBzS 2 ) was prepared by dissolving 0.101 mmol (18.5 mg) of MeBzS 2 and 0.099 mmol (17.8 mg) of PdCl 2 in 10 mL of CH 3 CN, heating the obtained solution to the boiling point for 20 min, followed by hot filtration.The filtrate was left for slow evaporation in air at an ambient temperature, leading to the formation of orange-red crystals, from which a sample suitable for the XRD study was taken.The obtained solid material was washed with 1 mL of CH 3 CN and dried in air at the ambient temperature, giving an 85% yield (31 mg).
X-ray Diffraction.The XRD experiment was conducted at 100 K using a SuperNova diffractometer (Agilent Technologies, currently Rigaku Oxford Diffraction) equipped with an Atlas CCD detector and an Oxford Cryosystems cryogenic attachment.The Mo−Kα characteristic radiation (0.71073 Å) was used for the measurements, and data integration was performed using CrysAlis Pro software (v.1.171.38.41q).The structure of PdCl 2 (MeBzS 2 ) was solved with the SHELXS-2013 software via direct methods, and its subsequent refinement was carried out with the SHELXL-2018/3 software. 29Hydrogen atoms were treated as riding atoms with U iso (H) equal to 1.2U eq (C) or 1.5U eq (C) and attached in calculated positions.Supplementary crystallographic data for this article are accessible at no cost via the Cambridge Crystallographic Data Centre website under deposition number 2043225.
Broadband Dielectric Spectroscopy.The BDS technique was performed to reinvestigate the ambient-pressure dielectric response of MeBzS 2 .For this purpose, the sample was poured between two parallel stainless-steel plates of a capacitor, which were 10 mm in diameter and distanced by two quartz fibers of 100 μm thickness.The so-prepared capacitor was sealed by a Teflon ring and cooled to 118 K at a rate of approximately 15 K min −1 .Dielectric studies were performed under quasi-static conditions between 118 and 221 K, starting from the lowest temperature.Notably, during measurements up to 217 K (i.e., within the temperature at which the relaxation times were determined), the time needed to stabilize and maintain the sample at each temperature was shorter than the time needed to start crystallization, so the sample was kept in the supercooled liquid state.The measurements were performed with steps ΔT = 5 K below T g of MeBzS 2 or ΔT = 2 K for temperatures in the vicinity and above its T g utilizing the nitrogen gas and a Novocontrol Quattro system for temperature control and stabilization.Dielectric spectra were collected between 10 −1 and 10 6 Hz, utilizing a Novocontrol Broadband Dielectric Spectrometer equipped with an Alpha Impedance analyzer.
A different protocol was used, while the crystallization process was monitored with the BDS technique.In this case, the experiment started at room temperature, i.e., above the melting point of MeBzS 2 .In the first step, the liquid was cooled to 153 K (i.e., below its T g ) at a rate of roughly 20 K/min and kept at this temperature for 20 min.Subsequently, the sample was heated to the desired temperature at approximately 5 K/ min, where the kinetics of isothermal crystallization should be measured.These investigations were performed at 213, 215, 217, 219, 221, 223, and 225 K.The sample was changed after each experiment, and each measurement was repeated twice for each temperature condition to check the results repeatability.
The collected data were subjected to the analysis in the domain of the complex dielectric permittivity: ε* = ε′ − iε″, where ε* is the complex dielectric permittivity, whereas ε′ and ε″ are its real and imaginary parts, respectively.The analysis was conducted with WinFit software.
DFT Calculations.DFT 30,31 at a hybrid B3PW91 level of theory 32 and 6-311++G(d,p) basis set 33−35 was employed to optimize the molecular geometry of MeBzS 2 with minimum energy (conformer M1).The molecule with adopted atom labeling is presented in Figure 1a.The optimized structure was used as the input file to calculate the vibrational frequency to confirm its identity as an energy minimum.Then, a comprehensive conformational analysis was performed with ±5°step size for five dihedral angles within a heterocycle ring, recording both energy and dipole moment vector variations (see Figure S1 in Supporting Information).All calculations were performed in the gas phase using the Gaussian 16, Revision C.01 software package in a single-molecule approach. 36 detailed analysis was conducted for the dihedral angles The influence of an oriented external electric field on the geometry and electronic properties of MeBzS 2 was examined at the same level of theory after prior transforming its geometry to the Z-matrix format according to the previous report. 37We utilized the structure of the M1 conformer optimized at the zero field as an input geometry for all the calculations.Notably, the transformation procedure changed its orientation with respect to the X, Y, and Z axes compared to the PES calculations (see Figure S2 in Supporting Information).The oriented external electric field was applied in the Gaussian 16 software along the X, Y, or Z axis by means of the "Field = M ± N" keyword, where M is the direction and N is the field magnitude expressed in atomic units. 37,38For example, the notation "Field = X + 10" adds to the Hamiltonian a potential term related to the electric field of 0.001 au (0.514 V/nm) 39 oriented along the X axis. 38Under such conditions, the Hamiltonian of a system can be expressed in the first approximation as where H ̂0 is the Hamiltonian without an external electric field, H ̂field is a term describing the interaction of a molecule with an electric field, μ̂is the dipole moment operator, μx, μŷ, μẑ are the x, y, and z components of the dipole moment operator, and F −42 All DFT calculations taking into account of the electric field were performed without any symmetry constraint, utilizing a series of gradients from −0.03 to 0.03 au, which are technically achievable. 38,43inally, we used a hybrid B3PW91 level of theory and diffuse (augmented) functions as in the aug-seg-cc-pVTZ-PP basis set 44,45 on palladium and 6-311++G(d,p) basis set 33−35

■ RESULTS AND DISCUSSION
Our new broadband dielectric measurements confirm the occurrence of both structural α relaxation and a secondary β process for MeBzS 2 (Figure 1b−d).Both dielectric relaxations shift toward higher frequencies as the temperature increases, which is related to the progressive shortening of the corresponding relaxation times (τ α , τ β ) according to the relation In this expression, f max is the frequency related to the maximum of the relaxation peak.To determine precisely f max (and thus τ α and τ β ), the α relaxation was parameterized with the Havriliak−Negami function (see Figure 1c). 46In contrast, the β process was described with the Cole−Cole function following the previously described procedures (Figure 1d). 24,47he relaxation times τ α and τ β were calculated from the fit parameters according to the expression where τ HN is the Havriliak−Negami relaxation time and α HN and β HN are shape parameters describing the dispersion of dielectric relaxation in the complex dielectric permittivity ε*(f) domain. 5As shown in Figure 1b, τ β in its logarithm form changes in a linear way with the temperature inverse, obeying the Arrhenius law In this expression, E a , R, and τ 0 are the activation energy, the gas constant, and the pre-exponential factor determining the relaxation time at the limit of T → ∞, respectively.The best fits are obtained when the parameters E a and log 10 τ 0 are equal to 10.7 ± 0.5 kJ/mol and −10.7 ± 0.1, respectively, which agrees with the previous study on this compound. 24−50 In this case, the best fits are obtained for B, and T 0 equals to 3540 ± 140 K and 124.2 ± 1.5 K.A characteristic feature of this dependence is that the relaxation peak amplitude starts abruptly decreasing when τ α is close to 3.4 μs, i.e., around 217 K.This phenomenon is well reflected by the temperature−time dependence of ε″, which was obtained by transforming the classic ε″(f, T) dielectric spectra into ε″(τ,1000/T) map based on formula 2 and color coding of dielectric losses (Figure 1b).According to the previous report on this compound, the diminishing relaxation peak amplitude corresponds to the cold crystallization of MeBzS 2 from its supercooled liquid state. 24Noteworthily, under these temperature conditions, the apparent activation energy of the α relaxation process, E a,α , ranges between ∼140 and ∼160 kJ/ mol (see the inset in Figure 1b).This physical quantity has been determined based on τ α (T −1 ) dependence according to the formula 5 To shed more light on the nonequilibrium near-T g cold crystallization process in MeBzS 2 , we use the BDS technique to study the kinetics of isothermal crystallization between 213 and 225 K.
In general, crystallization is a multistep process, which encompasses the formation of prenucleation aggregates (i.e., self-assemblies of structure similar to the one occurring in the crystal phase), formation of crystal nuclei, and subsequent crystal growth. 6,51,52Due to its complex character, crystallization can be affected by a vast array of factors, such as the thermodynamic history of the sample, temperature, or electric field. 6,53Therefore, strictly following an adopted experimental protocol for the measurement of crystallization kinetics is essential.In the case of MeBzS 2 , the experiment started at room temperature, i.e., above the melting point of this compound.In the first step, the liquid has been cooled down to 153 K (which is below T g ) with a rate of roughly 20 K/min and kept at this temperature for 20 min.Subsequently, the sample was heated at a rate of approximately 5 K/min to the desired temperature, at which the kinetics of isothermal crystallization should be measured.This investigation has been performed at 213, 215, 217, 219, 221, 223, and 225 K with a constant time step.The sample was changed after each experiment, and measurements were repeated twice for each temperature condition to check the results repeatability.
Figure 2a,b exhibits representative frequency-dependent dielectric ε′( f) and ε″(f) spectra collected while isothermal crystallization of MeBzS 2 at 213 K.During this process, both static dielectric permittivity (ε s ) and relaxation peak amplitude gradually diminish with time, leading to a complete disappearance of the structural relaxation after approximately 360 min.This phenomenon is related to the increasing crystallinity of the system and, consequently, the decreasing number of relaxing dipoles (molecules) due to their immobilization in the crystal structure where μ denotes the permanent dipole moments of relaxing molecules and N is their total number per unit of volume. 6ccording to this formula, the total vanishing of the structural relaxation is related to 100% conversion of the supercooled liquid to the crystal phase.Moreover, this expression shows that the real and imaginary parts of dielectric permittivity are coupled, and thus, their analysis delivers the same information about dielectric relaxation processes.Hence, to study the kinetics of isothermal crystallization, we will focus solely on the changes in the real part of the dielectric permittivity.In this case, the time-related changes in the crystal volume fraction (V cryst ) concerning the total volume (V total ) of the system are defined in eq. 8 where ε′ n is the so-called normalized real permittivity, ε′(0) is the initial static dielectric permittivity (i.e., the static dielectric permittivity of the liquid phase at given temperature−pressure conditions), ε′(t) is the static dielectric permittivity at time t, and ε′(∞) is its value in the long-time limit. 6Figure 2c depicts the time dependences of ε′ n measured between 215 and 225 K for MeBzS 2 .As can be seen, the crystallization kinetic curves adopt the characteristic S shape independently of the experimental conditions.However, the isothermal crystallization of MeBzS 2 from its supercooled liquid state slows down with the decreasing temperature from 225 to 215 K. To quantify these changes, we fit the experimental data with the Avrami model n n 0 (9)   where k is the crystallization rate constant, t 0 is the induction time of crystallization, and n is the so-called Avrami exponent, which is related to the dimensionality of the crystallites. 54,55In the case of MeBzS 2 , we neglect the variable t 0 (assume that t 0 = 0 s), which has been reported not to introduce any significant error to the analysis if the crystallization of a system is fast. 56s shown in the inset of Figure 2c, such an approach is sufficient to describe the experimental data.All parameters characterizing the isothermal crystallization of MeBzS 2 between 215 and 225 K obtained from the Avrami equation are collected in Table 1.
According to them, the parameter n is equal to 3 independently of temperature conditions (Figure 2d).In turn, the characteristic time τ cryst decreases when the temperature increases from 215 to 225 K, which corresponds where k 0 is a fitting parameter and E a,cryst is the activation energy for the crystallization process.In the case of MeBzS 2 , E a,cryst takes the value of ∼46 kJ/mol, which is approximately three times lower than the activation energy E a,α of the structural relaxation process under comparable temperature conditions.Since the ratio between these quantities (E a,cryst / E a,α ) is precisely the same as the dimensionality of the crystallites (E a,cryst /E a,α = 3 = n), it seems plausible that the crystallization process may be controlled (or at least highly affected) by the molecular dynamics in the supercooled liquid.Apart from translations or reorientations of entire molecules, molecular dynamics of cyclic organic compounds also encompass conformational changes within their structure.Such intramolecular transformations are relevant to MeBzS 2 , and previous study on this compound has predicted two energetically favored conformers and an intermediate transient state. 24However, two isomeric transient conformations are possible due to a methyl CH 3 -substituent attached to the aromatic ring.To explore the conformational interconversion possibilities within MeBzS 2 in more detail, we carried out quantum DFT studies using a single-molecule approach and the 6-311++G(d,p) basis set.Our investigation involved the calculation of potential energy curves for dihedral angles As shown in Figure 3a, the PES of MeBzS 2 takes the shape of an elliptical double potential well with two energy minima, two saddle points, and a local energy maximum.The two privileged conformers, M1 and M2, are nearly equal in energy.They are characterized by the staggered arrangement of hydrogen atoms within the −CH 2 −CH 2 − bridge (Figure 3b).In this moiety, the carbon atoms are positioned on opposite sides of the aromatic ring plane, resulting in a half-chair geometry and a small dipole moment value of these conformers.In contrast, conformations T1* and T2* corresponding to the saddle points contain eclipsed hydrogen atoms in the −CH 2 −CH 2 − bridge (Figure 3b).Consequently, their defining characteristic is the φ S1−C1−C2−S2 dihedral angle, which is close to 0°.This spatial arrangement, featuring two exodentate sulfur atoms, allows for maximizing the dipole moment value.However, due to emerging stresses within the heterocyclic ring, transient conformations T1* and T2* are 10.2 kJ/mol higher in energy than M1 and M2.
The most energetically disfavored conformation of MeBzS 2 is T3*.It is roughly 52.5 kJ/mol higher in energy compared to M1 and M2 and features a coplanar alignment of all carbon and sulfur atoms (see Figure 3b).Consequently, the dihedral angles φ S1−C1−C2−S2 and φ C8−S1−C1−C2 are almost equal to 0°, which makes the structure highly stressed.Noteworthy is that this conformation would appear as a transient geometry during direct interconversion between M1 and M2 according to the hopping mechanism (see Figure 3c).Such a transformation is characterized by an energy barrier of 52.5 kJ/mol, which is much higher than the activation energy of the secondary   4d).The close correspondence between the energy barrier and E a , coupled with the substantial potential for spatial rearrangement of the dipole moment vector and significant fluctuations in its magnitude, strongly suggests that this mode of conformational interconversion represents the most likely source of intramolecular β relaxation in MeBzS 2 .To validate this hypothesis, we investigate the behavior of MeBzS 2 when exposed to an external electric field.First, we examine the response of the MeBzS 2 molecule to an oriented electric field using DFT calculations.For this purpose, we align conformer M1 so that its aromatic ring lies almost in the XZ plane and the sulfur atoms are aligned along the Z axis.As shown in Figure 5a, applying an electric field along the X or Z direction to such an oriented molecule does not change its conformation significantly, altering its bond lengths, angles, and torsion angles to a small extent.Only the dipole moment considerably changes its direction and magnitude with the electric field in this case (see Figure 5a,b,c).This phenomenon originates from the nonzero polarizability and hyperpolarizability tensors of MeBzS 2 and, consequently, the appearing induced dipole moment.Namely, the field dependence of the dipole moment vector in polar systems is described in eq. 12 where μ 0 is the permanent (inherent) dipole moment vector, α is the molecular polarizability tensor, and β and γ are the second-and third-order hyperpolarizabilities. 42The symbol F represents the external electric field vector. 42As indicated with the red line in Figure 5b and the green curve in Figure 5c, the field dependence of the dipole moment value in MeBzS 2 exposed to the electric field oriented along the X or Z The Journal of Physical Chemistry B direction can be sufficiently described by the following approximation of eq. 12, in which the hyperpolarizability tensors are neglected where d = x or z denotes the specific direction (X or Z), μ 0x = −0.5974D,μ 0y = 0.0688D, and μ 0z = −0.1825Dare the x, y, and z components of the permanent dipole moment vector determined for the optimized zero-field geometry M1, whereas , and α zz = 24.0676× 10 −40 C 2 m 2 J −1 are the components of the polarizability matrix of the optimized zero-field structure M1.Significant changes in the dipole moment value also occur when the electric field is applied along the Y axis on conformer M1 (Figure 5d).However, in this case, these changes result primarily from field-induced modifications in the geometry of MeBzS 2 (cf. Figure 5a,d).Namely, as shown in Figure 5d, there is a significant discrepancy between the calculated dipole moment values (red dots in Figure 5d) and predictions made according to eq. 13 with d = y and the following components of the polarizability matrix of the optimized zero-field structure M1: α xy = 0.1635 × 10 −40 C 2 m 2 J −1 , α yy = 14.5925 × 10 −40 C 2 m 2 J −1 , and α zy = 0.1611 × 10 −40 C 2 m 2 J −1 (blue line in Figure 5d).The DFT calculations suggest that even small electric fields along the Y axis can induce conformational transformations.For instance, applying the oriented electric field along the −Y-axis direction causes the dihedral angle φ C8−S1−C1−C2 to increase nonlinearly to roughly 73°(Figure 5e).In turn, increasing the electric field along the +Y-axis direction up to 3 × 10 −3 au (1.542 V/nm) gradually decreases this dihedral angle value to approximately −2.8°.Notably, the geometry of MeBzS 2 deviates significantly from a half-chair conformation below −1.5 × 10 −3 au (0.771 V/nm) and above +1.8× 10 −3 au (0.925 V/nm) when the electric field is applied along the Y axis.As illustrated in Figure 5f  The Journal of Physical Chemistry B entire molecules are frozen, as in the glass phase, the orientational polarization may indeed arise from conformational changes following this interconversion path.This statement is also reinforced when concerning the thermal energy delivered to the condensed system, which does not exceed 1.6 kJ/mol for the glass phase (T < T g = 192 K).This value is sufficient to change the dihedral angles φ S1−C1−C2−S2 and φ C8−S1−C1−C2 to approximately −65 and 57°, respectively (Figure 5g).According to the DFT calculations, it is possible to change the conformation of MeBzS 2 significantly under such conditions by applying electric fields not exceeding 1 × 10 −5 au (i.e., less than 5.14 × 10 6 V/m), which are easily applicable in the dielectric spectroscopy technique (see the red arrow in Figure 5d,e,f,g). 57hat is more, transient conformers are energetically favored over conformer M1 in the presence of an external electric field oriented along the Y axis (see Figure 5h).This feature can be explained by utilizing the Taylor expansion of the molecular system energy under an electric field 42 where d, d′, and d′′ denote the specific components (x, y, or z) of the corresponding vectors or tensors.According to this expression, the energy E 0 of a specific conformer becomes corrected under the electric field F by terms containing permanent dipole moment μ 0 , polarizability α, and hyperpolarizabilities β. 42 Minimizing E at a specific electric field F requires maximization of the μ 0 value and collinear alignment In general, the complexation of metal cations by organic ligands can occur in a solution.Under these conditions, the unreacted free chemical individua remain in thermal equilibrium with the formed complex salts of various geometries, which is quantified by a stability constant. 58ccording to the DFT calculations, the complexation of Pd 2+ by MeBzS 2 in geometries close to conformers M1 or M2 is ineffective and leads to unstable higher-energy structures (Figure 6a).In turn, transient conformations T1* and T2* are the most energetically privileged geometries of MeBzS 2 in the presence of PdCl 2 (Figure 6a).Their preferred occurrence can be ascribed, among others, to charge−dipole interactions.

The Journal of Physical Chemistry B
Namely, ions are the source of the local electric field, and any polar objects (i.e., possessing a permanent electric dipole moment) are subject to torque when placed in an external electric field. 59,60The torque makes the dipoles align parallel with the field so that the potential energy is minimized and the attractive Coulomb interactions with ionic species are the most effective.Eq. 15 defines the standard Coulomb energy describing the charge−dipole interaction ) where ε 0 is the vacuum permittivity, q is the charge on an ion, μ is the dipole moment of an organic molecule, and |r qμ | is the center of mass distance between the charge and the molecular dipole. 59  These possible angular fluctuations correspond to small displacement of atoms S1, S2, C1, and C2, which do not exceed 0.03, 0.03, 0.10, and 0.09 Å, respectively (Figure 6e).Consequently, one can expect a well-ordered structure of the complex salt PdCl 2 (MeBzS 2 ) in its crystal phase with relatively small thermal ellipsoids.To prove this hypothesis, we performed XRD measurements.Complex salt PdCl 2 (MeBzS 2 ) crystallizes in a monoclinic system with space group P2 1 /c (see Table S1 for more details).As depicted in Figure 6f, the unit cell contains four molecules of the salt aligned in a way that allows the formation of intermolecular interactions and Cl•••H.The distance between centroids of interacting benzene rings via π•••π stacking is equal to 4.035 Å.In turn, the Cl•••H distances across their short contracts take the following values: 2.790 Å for Cl1 Each Pd 2+ cation (Pd1) in the complex salt is coordinated by two sulfur atoms, S1 and S2, of a single heterocyclic organic ligand and two chlorine ions, Cl1 and Cl2 (Figure 6g).The distances between Pd 2+ cation and the coordinating individua S1, S2, Cl1, and Cl2 are equal to 2.261(3), 2.286(3), 2.293(3), and 2.306(3) Å, respectively.The heterocyclic organic ligand MeBzS 2 adopts a highly stressed and energetically unfavorable conformation in its complex salt with the opposite conformation of hydrogen atoms, and both carbon atoms of the ethylene bridge −CH 2 −CH 2 − are located on the same side of the aromatic ring plane.Noteworthy is that this architecture closely resembles the geometry of the transition state T1*, which agrees with previous predictions made by DFT calculations.The value of the dihedral angle φ S1−C1−C2−S2 deviates from 0°to a small extent, being equal to 7(1)°.This value falls within the range of (−12.5°,11.4°), which is previously predicted by DFT.It also agrees with the anticipated small atom displacements with respect to the conformer T1*.Consequently, it is likely that the observed small deviation from T1* geometry results from thermal fluctuations.However, intermolecular interactions occurring in the crystal structure should also be taken into account.
The direct observation of the transient geometry close to T1* supports our previous predictions that conformational changes in MeBzS 2 can occur according to the scheme M1 → [T2*] → M2 → [T1*] → M1.It also reinforces our conclusion that the β relaxation of MeBzS 2 is related to the intramolecular conformational dynamics (not an in-plane rotation of molecules).The initially energetically disfavored transient geometries become energetically privileged after the electric field is applied or in the presence of ions.This phenomenon may even induce an unprecedented inversion of states in which all of the MeBzS 2 molecules are transformed from the M1 or M2 geometries to the transient conformer T1* or T2*, as observed in the crystal structure of PdCl 2 (MeBzS 2 ).

■ CONCLUSIONS
MeBzS 2 is a moderately fragile heterocyclic glass-former with a considerable tendency toward crystallization from the supercooled liquid state.Its cold crystallization is characterized by activation energy equal to 46 ± 5 kJ/mol and is profoundly influenced by molecular dynamics.In terms of dielectric response, this heterocycle is characterized by the structural α relaxation and a secondary β process, which offer insights into dynamics on various molecular scales.The first process is associated with cooperative reorientations of entire molecules in the supercooled liquid.In turn, the dielectric β relaxation has intramolecular character for MeBzS 2 , originating from mutual interconversions between two energetically favored conformers with half-chair geometries (M1 and M2).Notably, these conformational changes do not adhere to a direct hopping mechanism between these conformers.Instead, they proceed through two transient geometries, T1* and T2*, corresponding to the saddle points of the elliptical-shaped PES of MeBzS 2 .These intramolecular transformations induce substantial alterations in both dipole moment orientation and magnitude.The highest dipole moment value is observed for the transient conformers, which contain eclipsed hydrogen atoms in the −CH 2 −CH 2 − bridge and two exodentate sulfur atoms.Consequently, the initially energetically privileged conformations M1 and M2 in an undisturbed system become less favored after the electric field is applied or in the presence of ions.This phenomenon may even lead to an unprecedented The Journal of Physical Chemistry B inversion of states in which all of the MeBzS 2 molecules are transformed to the transient conformer T1* or T2*.The intriguing behavior has been mathematically rationalized and confirmed by the crystal structure of the complex salt PdCl 2 (MeBzS 2 ), in which the transient conformer has been directly observed.

Figure 1 .
Figure 1.(a) Molecule of MeBzS 2 with adopted atom labeling.(b) Relaxation times τ α and τ β of MeBzS 2 plotted versus temperature inverse and superimposed on temperature−time dependence of dielectric losses ε″.The values of ε″are coded by colors.The inset shows temperature-induced changes in activation energy for the α process.(c) Structural α relaxation visible in frequency-dependent ε″(f) and ε′( f) spectra measured at 197 K.The red line shows parameterization of experimental data with the Havriliak−Negami function.(d) Representative frequency-dependent ε″( f) and ε′(f) spectra measured at 123 K with the apparent secondary β process (black symbols), parameterized with the Cole−Cole function (red line).

Figure 2 .
Figure 2. Dielectric ε′(f) (a) and ε″(f) (b) spectra registered during isothermal crystallization of MeBzS 2 at 213 K. (c) Time dependence of the normalized real permittivity during the isothermal crystallization of MeBzS 2 between 215 and 225 K.(d) Temperature dependence of Avrami exponent n.(e) Temperature evolution of crystal growth rate k fitted to the Arrhenius law.
, the field-induced conformational changes in MeBzS 2 closely resemble those observed for the interconversion path M1 → [T2*] → M2 → [T1*] → M1, particularly in terms of the variation in dihedral angles within the heterocyclic moiety.It means that if the

Figure 5 .
Figure 5. (a) Orientation of the MeBzS 2 molecule in the Cartesian coordinate system as well as field-induced changes in the MeBzS 2 geometry and its dipole moment vector orientation.(b) Changes in the dipole moment of MeBzS 2 under the applied electric field oriented along the X axis (black dots) and related fits according to expression (13).(c) Total dipole moment of MeBzS 2 versus the applied electric field oriented along the Z axis (black dots) fitted with function (13).(d) Field-induced changes in the dipole moment of MeBzS 2 related to the induced dipole moment (blue curve) and conformational changes (red dots) when applying the electric field along the Y axis.(e) Field-induced changes in the dihedral angle φ C8−S1−C1−C2 of MeBzS 2 when exposed to the electric field oriented along the Y direction.(f) Field-induced conformational interconversion path (red dots) superimposed on the sequence M1 → [T1*] → M2 → [T2*] → M1.(g) Possible conformational changes in MeBzS 2 under low electric fields.(h) Stabilization of transient conformers of MeBzS 2 under an external electric field oriented along the Y direction.

of μ 0
and F vectors.Maximizing μ 0 can be achieved by transforming MeBzS 2 into the strained intermediate conformation T1* or T2*.Consequently, one might even expect field-induced induction of the transient geometries T1* and T2* for a specific alignment of MeBzS 2 molecules with respect to the electric field lines.One can conclude that the substantial potential for temperature-and field-induced spatial rearrangement of the dipole moment vector in MeBzS 2 , the related significant fluctuations in its magnitude, the ease of fieldinduced conformational changes within the heterocyclic ring, the field-induced stabilization of transient geometries, and the previously mentioned close correspondence between the energy barrier and activation energy E a clearly indicate that the interconversions following the scheme M1 → [T2*] → M2 → [T1*] → M1 indeed occur in the MeBzS 2 and are responsible for its secondary β relaxation.To support experimentally our theoretical predictions, we investigate the behavior of MeBzS 2 in the presence of metal cations and synthesize the complex salt PdCl 2 (MeBzS 2 ).

Table 1 .
Parameters Characterizing the Isothermal Crystallization Kinetics of MeBzS 2 Obtained from the Avrami Model 2.97 ± 0.15 225 (5.18 ± 0.45)•10 −4 3.03 ± 0.12The Journal of Physical Chemistry B to a gradually growing value of crystallization rate constant k in this temperature range (cf.Table1and Figure2e).To dissect this dependence, we parameterize the log 10 k = f(T −1 ) dependence with the Arrhenius law Interaction between Pd 2+ and lone pairs of S atoms from MeBzS 2 is ineffective for conformers M1 and M2 because of the considerable angle between vectors r qu and μ.Maximizing the Coulomb energy (in terms of its absolute value) requires transforming MeBzS 2 to the strained intermediate conformer T1* (or T2*), characterized by the highest dipole moment value and almost collinear alignment of vectors r qu and μ.Considering the attractive nature of the Pd 2+ −MeBzS 2 interactions, the entire energy of the forming PdCl 2 (MeBzS 2 ) is minimized in this way.Indeed, as shown in